Facetime can't call from iPhone/iPad to iMac - Busy Tone Always (Connection Lost)

I'm not sure if this is due to update to iOS 7, but here is the issue (which I tried to be as precise & detail as possible):

- I have set up Facetime on all of my 3 devices - iMac (latest 27" iMac with OSX 10.8.5), iPad 3 & iPhone 5 (both updated to iOS 7)

- For iMac, I use my AppleID email #1, and I have no problem activating it

- For iPad, I use my another AppleID email #2, and I have no problem activating it. And I have unchecked my phone number. So only AppleID email #2 is checked

- For iPhone, I also use AppleID email #2, and I have no problem activating it. And here I checked only my phone number, and unchecked AppleID email #2

- I can make Facetime call from iMac to both iPad & iPhone, but I cannot make Factime call from iPad or iPhone to iMac, nor from iPhone to iPad, nor iPad to iPhone - always busy tone (it says connection lost)

- I have tried switching off Facetime on all devices, rebooting and re-activating. No luck

- I have also tried resetting & restoring my iPad and re-activating Facetime. Also no luck - I didn't bother to reset/restore my iPhone after I tried that on my iPad

- My iMac is connected to internet via ethernet with no firewall. My iPad & iPhone are connected to internet via Wi-Fi (from the same modem as the ethernet)

- I also noticed that before iOS 7, there was always a Facetime icon next to each of my phone number & AppleID emails, so that I know which one I can make Facetime call with. But after iOS 7, there becomes a line called Facetime with Facetime icon between phone numbers and emails on the address book. And say if I press one of the AppleID emails from my iPhone Facetime, it doesn't attempt to make Facetime call, but simply switch me to email or message.

Seems like a very weird problem. Not sure if others share the same experience.

Please help. Many thanks in advance.

Gary